Display device

ABSTRACT

The invention provides a method and apparatus to obtain accurate gradation by obtaining an accurate ratio of light emitting parts in a display device which implements gradation by forming a plurality of TFTs and a plurality of OELDs in each pixel, directly connecting the TFTs and OELDs, and switching an on and off state of the TFTs, and controlling a light emitting area of the OELDs. A plurality of OELDs have the same shape, and gradation can be implemented by controlling the number of OELDs that emit light. A plurality of OELDs have a round shape. A plurality of OELDs are arranged at the same interval in a vertical and/or horizontal direction. According to this structure, because the light emitting areas of the plurality of OELDs become equal to each other, by controlling the number of OELDs, a ratio of the light emitting areas can be accurately obtained.

This is a division of application Ser. No. 09/468,356 filed Dec. 21,1999, now U.S. Pat. No. 6,225,750. The entire disclosure of the priorapplication is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to a display device, particularly a thin filmtransistor driven organic electro-luminescent display device (hereafterreferred to as TFT-OELD) which is driven by a thin film transistor(hereafter referred to as TFT) and provided with an organicelectro-luminescent element (hereafter referred to as OELD) of a highpolymer system formed in a liquid phase process.

2. Description of Related Art

TFT-OELD is promising because it is a display device which realizeslightweightness, thinness, smallness, higher accuracy, wider view angle,lower electric consumption, and the like. FIG. 1 shows a conventionalTFT-OELD. FIG. 2 shows a cross-sectional view of the conventionalTFT-OELD. Here, there is only one pixel 11 depicted, but there areactually many pixels 11 in plural rows and lines. Here, OELD 18 is ahigh polymer system, formed by a liquid phase process, such as spincoating, blade coating, ink jet, or the like.

In order to implement gradation, in the case of the structure shown inFIG. 1, a gate voltage of a driving TFT 17 is made to change andconductance is changed, so electric current which flows in the OELD 18needs to be controlled. However, gradation according to this method,particularly in half tone, result in irregularity of transistorcharacteristics of the driving TFT 17 and appears as brightnessirregularity of the OELD 18, and there is a problem such that the screenbecomes non-uniform.

Therefore, as shown in FIG. 3, a method is considered which implementsgradation by changing a light emitting area of the OELD 18 (JapanesePatent Application 9-233107). FIG. 4 shows a driving method of thismethod. A scanning electric potential 31 is applied to a scanning line12, and a signal line 13 is formed of a signal line (lower bit) 131 anda signal line (upper bit) 132. A signal electric potential (lower bit)321 and signal electric potential (upper bit) 322 are respectivelyapplied as a signal electric potential 32. A driving TFT 17 is formed ofa driving TFT (lower bit) 171 and a driving TFT (upper bit) 172, and theOELD 18 is formed of an OELD (lower bit) 181 and an OELD (upper bit)182. In this example, 2-bit 4 gradation is considered, so an area ratiobetween OELD (lower bit) 181 and OELD (upper bit) 182 is 1:2.

In this method, the driving TFT 17 takes either a substantiallycompletely on state or a substantially completely off state. In the onstate, the resistance of the driving TFT 17 is small enough to beignored, compared to the resistance of OELD 18, and the electric currentamount which flows in the driving TFT 17 and OELD 18 is substantiallydetermined by only the resistance of the OELD 17.

Therefore, irregularity of transistor characteristics of the driving TFT17 does not appear as brightness irregularity of the OELD 18.Furthermore, in the off state, voltage applied to the OELD 18 becomesless than a threshold voltage, so the OELD 18 does not emit light atall, and, needless to say, irregularity of transistor characteristics ofthe driving TFT 17 does not appear as brightness irregularity of theOELD 18.

FIG. 5 is a cross-sectional view of TFT-OELD which implements gradationdisplay by changing a light emitting area of the OELD 18 shown in FIGS.3 and 4. FIG. 5(a) is a cross-sectional view of the OELD (lower bit)181, and FIG. 5(b) is a cross-sectional view of the OELD (upper bit)182. The ratio between the light emitting part 25 of the OELD (lowerbit) 181 and the light emitting part 25 of the OELD (upper bit) 182 ispreferably 1:2.

A light emitting layer 22 is an OELD of a high polymer system and formedin a liquid phase process. A surface of a bank 24 is lyophobic and thelight emitting layer 22 does not remain. Therefore, the area of the OELD18 is determined by patterning. With respect to a side surface of thebank 24, the materials and processing determine whether the side surfaceof the bank 24 becomes lyophobic or lyophilic.

FIG. 5 shows the case of a lyophilic side surface of the bank 24. As aphenomenon that is characteristic of a liquid phase process, the lightemitting layer 22 has a cross-sectional shape which is pulled toward theside surface of the bank 24. In this case, electric current flows into athinner part of the light emitting layer 22, and this part becomes alight emitting part 25. The cross-sectional shape of the light emittinglayer 22 described here is sensitive to liquid amount, liquid material,an initial position of the liquid, and a state, temperature, atmosphere,or the like of a substrate, and which are difficult to control. That is,it is difficult to obtain an absolute value of a desired light emittingarea. Because of this, it is difficult to obtain an accurate ratio of1:2, between the light emitting part 25 of the OELD (lower bit) 181 andthe light emitting part 25 of the OELD (upper bit) 182, and ultimately,it is difficult to obtain accurate gradation.

FIG. 6 is a cross-sectional view of OELD (lower bit) 181 (FIG. 6(a)) anda cross-sectional view of OELD (upper bit) 182 (FIG. 6(b)) in the samemanner as in FIG. 5. In FIG. 6, the side surface of the bank 24 islyophobic. As a phenomenon that is characteristic of a liquid phaseprocess, the light emitting layer 22 has a cross-sectional shape whichis distant from the side surface of the bank 24. In this case as well,electric current flows into the thinner part of the light emitting layer22, and this part becomes the light emitting part 25. In this case aswell, in the same manner as in the case of FIG. 5, it is difficult toobtain an accurate ratio of 1:2 between the light emitting part 25 ofthe OELD (lower bit) 181 and the light emitting part 25 of the OELD(upper bit) 182, so it is difficult to obtain accurate gradation.

SUMMARY OF THE INVENTION

Therefore, one aspect of this invention is to obtain an accurate ratioof the light emitting parts, and accurate gradation. Therefore, theinvention may provide a display device in which gradation is implementedby forming a plurality of TFTs and a plurality of OELDs in each pixel,directly connecting the TFTs and OELDs, switching an on and off state ofthe TFTs, and controlling an area of the OELDs, that emits light,wherein the plurality of OELDs have the same shape, and gradation isimplemented by controlling the number of OELDs that are created to emitlight and by controlling an appropriate on/off state of the TFTconnected to each OELD.

According to this structure, as a characteristic phenomenon of a liquidphase process, even if an OELD becomes a cross-sectional shape which ispulled in to a side surface of a bank or is distant from the sidesurface of the bank, the light emitting part of each OELD is the samearea, and accurate gradation can be obtained. In this structure as well,it is difficult to obtain an absolute value of a desired light emittingarea, but the light emitting area of a plurality of OELDs becomes equal,so the ratio of the light emitting areas can be accurate by controllingthe number OELDs.

The display device may also include a plurality of OELDs that have around shape. According to this structure, the light emitting part ofeach OELD can reliably be the same area, and accurate gradation can beobtained. The reasons are as follows. When an OELD has a shape with arectangular vertex (or vertices), there is a possibility that aphenomenon may occur, for example, that the vertex becomes pulled in orthe vertex cannot be filled. This phenomenon prevents a user fromobtaining accurate gradation for the same reason as in the problems of across-sectional shape as described above. This phenomenon is moresensitive to the liquid amount, liquid material, initial position ofliquid, and the state, temperature, and atmosphere of a substrate, moreso than the problems in a cross-sectional shape described above, and itis difficult to control this phenomenon between adjacent OELDs. Bymaking the OELD round shaped, this phenomenon can be avoided.

The display device may also include a plurality of OELDs are arranged atthe same interval in a horizontal and/or vertical direction. Accordingto this structure, the light emitting part of each OELD is made to bemore reliably the same area, and accurate gradation can be obtained. Thereasons are as follows. When OELD is formed by spin coating or bladecoating,- the light emitting layer which is coated over all the pixels,due to the lyophobicity of a surface of the bank, the light emittinglayer naturally flows into a convex part of the bank. In the case ofinkjet as well, this may sometimes happen. At this time, when a concavearea surrounded by a bank convex part is large, the light emitting layercoated over this part flows into a convex bank portion, so the lightemitting layer becomes thick. When the convex area surrounded by thebank concave part is small, the light emitting layer becomes thin.Ultimately, irregularity of film thickness of the light emitting layeris generated. This irregularity can be avoided by arranging a pluralityof OELDs at the same interval in a horizontal or vertical direction.

Additionally, according to this structure, when the OELDs are formed byan ink jet process, ink jetting can be performed at the same interval,so fabrication can be simplified.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a pixel of conventional TFT-OELD whichimplements a gradation display by changing a TFT conductance;

FIG. 2 is a cross-sectional view of a conventional TFT-OELD;

FIG. 3 is a diagram showing one pixel of a TFT-OELD which implements agradation display by changing a conventional OELD light emitting area;

FIG. 4 is a diagram showing a TFT-OELD driving method which implements agradation display by changing a conventional OELD light emitting area;

FIGS. 5(a) and 5(b) are cross-sectional views of an OELD in a TFT-OELDwhich implements a gradation display by changing a conventional OELDlight emitting area (when the side surface of the bank is lyophilic),where FIG. 5(a) is a diagram of an OELD (lower bit) and FIG. 5(b) is adiagram of an OELD (upper bit);

FIGS. 6(a) and 6(b) are cross-sectional views of an OELD in a TFT-OELDwhich implements a gradation display by changing a conventional. OELDlight emitting area (when the side surface of the bank is lyophobic),where FIG. 6(a) is a diagram of an OELD (lower bit), and FIG. 6(b) is adiagram of an OELD (upper bit);

FIG. 7 is a diagram showing one pixel of a TFT-OELD related toembodiment 1 of this invention;

FIG. 8 is a diagram showing one pixel of a TFT-OELD related toembodiment 2 of this invention; and

FIG. 9 is a diagram showing one pixel of a TFT-OELD related toembodiment 3 of this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following explains preferred embodiments of this invention, basedupon the drawings.

FIG. 7 is a diagram showing a pixel of a TFT-OELD related to a firstembodiment of this invention. Here, only one pixel 11 is depicted, butmany pixels 11 actually exist in a plurality of rows and a plurality oflines.

An OELD (lower bit) 181 is formed of OELD (lower bit·rectangular) 1811,and an OELD (upper bit) 182 is formed of OELD (upperbit·first·rectangular) 18211 and an OELD (upper bit·second·rectangular)18221. The OELD (lower bit·rectangular) 1811, the OELD (upperbit·first·rectangular) 18211, and the OELD (upperbit·second·rectangular) 18221 have the same shape, so the same lightemitting area can be obtained, and accurate gradation can be obtained bychanging the number of OELDs that are caused to emit light.

FIG. 8 is a diagram showing a pixel of a TFT-OELD related to anotherembodiment of this invention. Here, only one pixel 11 is depicted, butmany pixels 11 actually exist in a plurality of lines and a plurality ofrows.

An OELD (lower bit) 181 is formed of an OELD (lower bit-round shape)1812, and an OELD (upper bit) 182 is formed of OELD (upperbit·first·round shape) 18212 and OELD (upper bit·second·round shape)18222. The OELD (lower bit-round shape) 1812, the OELD (upperbit-first-round shape) 18212, and the OELD (upper bit·second·roundshape) 18222 have the same round shape, so the same light emitting areacan be reliably obtained, and accurate gradation can be obtained.

FIG. 9 is a diagram showing a pixel of a TFT-OELD related to anotherembodiment of this invention. Here, only one pixel 11 is depicted, butmany pixels 11 actually exist in a plurality of lines and a plurality ofrows.

An OELD (lower bit) 181 is formed of an OELD (lower bit·round shape)1812, and an OELD (upper bit) 182 is formed of an OELD (upperbit·first·round shape) 18212 and an OELD (upper bit·second·round shape)18222. The OELD (lower bit·round shape) 1812, the OELD (upperbit·first·round shape) 18212, and the OELD (upper bit·second·roundshape) 18222 are arranged at the same interval in horizontal andvertical directions within the pixel 11, and also with respect to theadjacent pixel 11. Because of this, the light emitting part of each OELDcan more reliably have the same area, and accurate gradation can beobtained.

Furthermore, as an EL element formed in each pixel, in first embodiment(FIG. 7), a rectangular element is shown as an example, and in thesecond and third embodiments (FIGS. 8 and 9, respectively), around-shaped element is shown, but this invention is not limited tothese. Accurate gradation can also be obtained in a polygonal-orelliptic-shaped element. In particular, an elliptic element, as in thecase of a round shape, does not have a vertex such as is present in thecase of a rectangular shape, so there is no problem such that the vertexcannot be filled by the light emitting layer.

Each of FIGS. 7-9 provide lower and upper TFTs 171, 172, respectively,comprising on/off states contributing to the control of gradation in thelower OELD (1811, FIG. 7; 1812, FIGS. 8 and 9) and upper OELDs (18211and 18221, FIG. 7; 18212 and 18222, FIGS. 8 and 9) each respective TFTcorresponds to.

As described above, according to this invention, by controlling the areaof an electro-luminescent element that emits light, accurate gradationcan be realized.

What is claimed is:
 1. A display device, comprising: a plurality oforganic electro-luminescent elements, at least one of the plurality oforganic electro-luminescent elements having light emitting areas thatare substantially the same as that of another of the plurality oforganic electro-luminescent elements, the plurality of organicelectro-luminescent elements being switchable between on and off states,the plurality of organic electro-luminescent elements emitting lightwhen in the on state; and a plurality of thin film transistors thatswitch the plurality of organic electro-luminescent elements between theon and off states, wherein gradation, for displaying, corresponds to anumber of the plurality of organic electro-luminescent elements that arein the on state.
 2. The display device according to claim 1, furtherincluding a bank, the light emitting area being defined by the bank. 3.The display device according to claim 2, the bank defining a surfacethat bears lyophobicity.
 4. The display device according to claim 1, ashape of the plurality of organic electro-luminescent elements beinground.
 5. The display device according to claim 1, the plurality oforganic electro-luminescent, elements being disposed at substantiallyregular intervals in a certain direction.
 6. A method for manufacturinga display device, comprising: forming a plurality of thin filmtransistors; forming a plurality of organic electro-luminescent elementsso that at least one of the plurality of organic electro-luminescentelements has light emitting areas that are substantially the same asthat of another of the plurality of organic electro-luminescentelements; and forming banks for defining the light emitting area of eachof the plurality of organic electro-luminescent elements.
 7. The methodaccording to claim 6, further comprising forming banks so that the lightemitting area is in a round shape.
 8. The method according to claim 6,the step of forming a plurality of organic electro-luminescent elementsincluding forming the plurality of organic electro-luminescent elementsat substantially regular intervals in a certain direction.
 9. The methodaccording to claim 6, the step of forming a plurality of organicelectro-luminescent elements including forming the plurality of organicelectro-luminescent elements at substantially regular intervals inhorizontal and vertical directions relative to the display device. 10.The method according to claim 6, the step of forming a plurality oforganic electro-luminescent elements including forming the plurality oforganic electro-luminescent elements at substantially regular intervalsin a horizontal direction relative to the display device.
 11. The methodaccording to claim 6, the step of forming a plurality of organicelectro-luminescent elements including forming the plurality of organicelectro-luminescent elements at substantially regular intervals in avertical direction relative to the display device.